The present invention relates to an apparatus and a process for encoding a gradation image in an image transmission system, and more particularly to a technique for reducing the amount of transmission data needed to represent the gradation image by encoding the transmission data, while preserving an edge component sensitive to human visual characteristics in a half tone image by considering the similarity between successive blocks of pixel data. Methods and apparatus for edge detection and correction are disclosed in the Peak et al., Edge Detection Method and Apparatus for an Image Processing System, (U.S. Pat. No. 5,212,740) and Park, Method and Circuit for Correcting Image Edge, (U.S. Pat. No. 5,151,787).
In image processing systems such as facsimiles, image scanners, printers, copiers, and the like, the expression of a half tone image is obtained from gradation image data from documents having gradations such as a picture, painting, etc. through the use of a dither matrix. The half tone image is classified into a real half tone and a dummy half tone, which differ in process of expression. Since the present invention deals with the dummy half tone, the expression process relating to the dummy half tone will be explained and no further description of the real half tone will be given. There are many processes of expressing the dummy half tone, including the ordered dither, random dither, error distribution, mean error least, mean difference limit, density pattern, and other processes. The ordered dither process is the simplest image expression process. The ordered dither process uses unvarying threshold locations in a matrix matched to entire images and determines a binarized threshold value from location information of each pixel. The ordered dither process is further divided into a dot concentration type such as spiral, scroll, etc. and a dot distribution type of a high spatial frequency such as bayer, dot, etc. One method for obtaining the half tone image expression from a gradation image quantized to a uniform gradation using the dither matrix is disclosed in the Method For Extracting Halftone Image Korea patent application No. 1990/16884, assigned to the instant assignee.
The half tone image expression using a dither matrix, however, is less than desirable because edges become smooth as a result of large brightness variation. That is, the human visual characteristic sensitive to edges is not considered, thus picture quality deteriorates. On the other hand, in transmitting the half tone image obtained by the dithering process using a dither matrix, encoding is needed. The encoding suitable for the half tone image however, has not been standardized in CCITT. Thus, in the encoding for transmitting the half tone image, a Modified Huffman or Modified READ encoding system of CCITT recommendation T.4 or Modified READ encoding system of T.6 has been used, as in the case of simple binarization data for documents of only characters. The Modified Huffman or Modified READ encoding system is suitable for a statistic characteristic obtained from the simple binarized image from one to eight of a CCITT test chart. Typically, since the Modified Huffman or Modified READ encoding system has similar or more data, transmission time increases.
Various encoding processes exist. I have observed that none however, effectively reduce the amount of transmission data needed for the reproduction of a gradation image while adequately preserving the edge component in the gradation image.
Pollich et al., Adaptive Clumped Dithering with Clump Plane Separation, (U.S. Pat. No. 5,243,446), mentions a process for data compression and image enhancement in a digital facsimile document transmission system. According to Pollich '446, low distinction blocks, containing low contrast information, are distinguished from high distinction blocks, containing high contrast information such as text or line drawing information. The low distinction blocks are encoded through clump plane separation techniques by clumping all of the pixels with corresponding threshold values together and encoding them. The high distinction blocks are not encoded, and are transmitted directly. The image is reproduced by re-integrating the clump planes to place the pixels of the low distinction block in their original positions. I have observed that this process does not effectively reduce the amount of transmission data needed to reproduce the image, because high distinction blocks, or edge portions, are not encoded and compressed, and are transmitted as is.
Miyaoka et al., Method for Image Data Coding, (U.S. Pat. No. 4,831,659), mentions a process for image data coding. According to Miyaoka '659, the global redundancy of an image is used. The image is divided into non-equal length blocks in accordance with changes in brightness. Portions exhibiting a gradual change in brightness are turned into large blocks and encoded. Portions exhibiting a sudden change in brightness are turned into small blocks and encoded. It has been my observation that this process does not effectively preserve detail in gradually changing portions of an image because the gradually changing portions are divided into large blocks and encoded. As a result, the gradually changing portions of an image are not preserved when the corresponding portions of the image are reproduced as a large block with continuous brightness.
Faul et al., Method for Encoding Color Images, (U.S. Pat. No. 5,247,589), discloses a process for encoding color images. According to the Faul '589, color image information is encoded by grouping adjacent pixels into blocks. Each block is compared with several templates to determine which template most closely approximates the block, and the block is encoded in accordance with the corresponding template. It seems to me however, that this template system results in loss of accuracy in transmitted data because the templates are only an approximation of each block of image data; thus, the transmitted data, which is made entirely of templates, is only an empirical approximation of the original image data.